1. Field of the Invention
The present invention relates to a method and apparatus for controlling the suspension of a vehicle. More particularly, this invention pertains to a method and apparatus which can automatically adjust the natural frequency of the suspension of a vehicle in such a direction as to suppress uncomfortable vibration of the vehicle caused by a misfire-originated torque fluctuation or in driving on ill-conditioned (uneven) roads.
2. Description of the Related Art
To improve comfortableness in riding, suspension control apparatuses have recently been put into practice which can automatically control the spring constant or damping force (natural frequency) of the suspension in accordance with the road conditions.
The suspension control apparatus of this type is provided with a sensor to detect the road conditions; for example, an ultrasonic vehicle height sensor for directly detecting the change in vehicle height with an ultrasonic wave or a stroke type vehicle height sensor for indirectly detecting the change in vehicle height based on the magnitude amount of shrinking/expanding movement of the suspension or an angular change in suspension arms. When unevenness of a road is detected by such a height sensor, the spring constant or damping force of the suspension is adjusted in accordance with, for example, the vehicle speed at that moment to maintain the comfortable riding or drivability on rugged roads.
In an arrangement utilizing the height sensor for detecting the road's unevenness as described above, however, such a sensor must be added for suspension control, thus disadvantageously increasing the manufacturing cost of the vehicle.
The above-described automatic control of the natural frequency (spring constant or damping force) of the suspension based on the detection of variation in vehicle height can suppress the degradation of comfortable riding due to fierce vehicle vibration caused by rugged roads. This control system however has a difficulty in accurately distinguishing the vehicle vibration on a relatively low-frequency side originated from a misfire in the internal combustion engine mounted on a vehicle from the one caused by other factors. That is, this system cannot provide the proper suspension control for a misfire-originated vehicle vibration. When the vehicle body vibrates due to a change in engine output torque caused by a misfire, the comfortable riding cannot be provided by the suspension control.